WO2006134787A1

WO2006134787A1 - Exhaust gas purifying catalyst

Info

Publication number: WO2006134787A1
Application number: PCT/JP2006/311106
Authority: WO
Inventors: Satoshi Matsueda; Mareo Kimura; Akimasa Hirai; Keiichi Narita; Hirohisa Tanaka; Mari Uenishi; Isao Tan; Masashi Taniguchi
Original assignee: Cataler Corporation; Daihatsu Motor Co., Ltd.
Priority date: 2005-06-16
Filing date: 2006-06-02
Publication date: 2006-12-21
Also published as: EP1913999A1; JP2006346587A; EP2698197A1; US8133839B2; US20090036300A1; EP1913999A4; CN101198404A; ZA200710440B; CN101198404B; JP4979900B2

Abstract

Disclosed is an exhaust gas purifying catalyst (1) containing a rare earth element, an alkaline earth element, zirconium and a noble metal. The atomic ratio of the alkaline earth element relative to the sum of the rare earth element and zirconium is not less than 10 at%. A part of the rare earth element and a part of zirconium form a complex oxide together with at least a part of the alkaline earth element, and this complex oxide and a part of the noble metal form a solid solution.

Description

Specification

Exhaust gas purification catalyst

Technical field

[0001] The present invention relates to an exhaust gas purifying catalyst.

Background art

Conventionally, as an exhaust gas purification catalyst for treating automobile exhaust gas, a three-way catalyst in which a precious metal such as platinum is supported on an inorganic oxide such as ceria or alumina has been widely used. Yes. In this three-way catalyst, the precious metal plays a role in promoting the reduction reaction of nitrogen oxides and the oxidation reaction of carbon monoxide and hydrocarbons. In addition, inorganic oxides play a role in increasing the specific surface area of the noble metal and suppressing the sintering of the noble metal by dissipating the heat generated by the reaction. In particular, ceria has oxygen storage capacity and can optimize the previous reduction and oxidation reactions.

[0003] Incidentally, in recent years, automatic propulsion vehicles such as automobiles have increased opportunities to travel at high speeds as their engine performance has improved. In addition, exhaust gas regulations to prevent air pollution are being strengthened. Against this backdrop, the exhaust gas temperature of self-propelled vehicles tends to become higher and higher.

[0004] Further, in order to suppress global warming, automatic propulsion vehicles are required to reduce carbon dioxide emissions. As a result, there are more opportunities to stop the fuel supply to the engine while the exhaust gas purification catalyst is heated to a high temperature.

[0005] That is, the exhaust gas purifying catalyst is used at a higher temperature as compared with the prior art, and the opportunity to be exposed to an oxygen-excess atmosphere at a high temperature is increasing. Therefore, there is a lot of research and development to realize an exhaust gas purifying catalyst that exhibits sufficient performance even in such a usage environment.

[0006] For example, Japanese Patent Application Laid-Open No. 5-168926, Japanese Patent Publication No. 6-75675, and Japanese Patent Application Laid-Open No. 2000-169148 disclose that the thermal stability of ceria is increased and its oxygen storage capacity is reduced. It is described to suppress this. Specifically, Japanese Patent Application Laid-Open No. 5-168926 discloses exhaust containing platinum group element, activated alumina, cerium oxide, norium compound and zirconium compound. A catalyst for gas purification is described. Japanese Examined Patent Publication No. 6-75675 discloses that the catalyst support layer contains cerium oxide, zirconium oxide, and catalytic metal, and at least a part of these cerium oxide and zirconium oxide is a complex oxide. Or, an exhaust gas purifying catalyst existing as a solid solution is described. JP 2000-169148 discloses a general formula: Ce

1 _

A cerium-based complex oxide represented by Zr Y 2 O is described.

+ b) a b 2-b / 2

[0007] Further, Japanese Patent Application Laid-Open No. 10-358 and Japanese Patent Application Laid-Open No. 2001-129399 describe that platinum sintering is suppressed by making platinum present as a platinum composite oxide. . Specifically, Japanese Patent Application Laid-Open No. 10-358 discloses a high heat-resistant composite oxide containing platinum and one or more elements selected from an alkaline earth metal element or a group IV element. A catalyst for exhaust gas purification using is described. In JP 2001-129399 A, a platinum complex oxide layer containing platinum element and an alkaline earth metal element is provided on an inorganic oxide support, and a metal X (X is Mg, Ca, An exhaust gas purifying catalyst is described in which an oxide layer of one or more elements selected from Sr, Ba, La and Ce is interposed.

[0008] However, if the thermal stability of the ceria is simply increased, platinum is sintered when the exhaust gas purification catalyst is exposed to an oxygen-excess atmosphere at a high temperature, for example, 1000 ° C to 1200 ° C. Thus, sufficient activity cannot be obtained. In addition, high-temperature firing is necessary to produce a platinum composite oxide with excellent thermal stability. For this reason, many exhaust gas purifying catalysts using platinum composite oxide have a small specific surface area and insufficient activity.

Disclosure of the invention

[0009] An object of the present invention is to provide an exhaust gas purifying catalyst which is less likely to cause a decrease in activity even when used in an atmosphere having a high temperature and a high oxygen concentration.

[0010] According to one aspect of the present invention, the rare earth element, the alkaline earth element, zirconium, and a noble metal are included, and the atomic ratio of the alkaline earth element to the sum of the rare earth element and zirconium is 10 atomic% or more. Part of the element and part of zirconium form a composite oxide with at least part of the alkaline earth element, and this composite oxide and part of the noble metal form a solid solution! / Provided exhaust gas purification catalyst.

Brief Description of Drawings

FIG. 1 is a diagram schematically showing an exhaust gas purifying catalyst according to an embodiment of the present invention. FIG. 2 is a diagram schematically showing a state change of the exhaust gas purifying catalyst of FIG. 1 under high temperature conditions.

FIG. 3 is a graph showing an X-ray diffraction spectrum of the powder produced in Example 1.

FIG. 4 is a TEM photograph of an exhaust gas purifying catalyst according to Example 2.

FIG. 5 is a graph showing changes in the X-ray diffraction spectrum accompanying changes in the atmosphere composition, obtained for the exhaust gas purifying catalyst according to Example 2.

FIG. 6 is a graph showing the change in solid solution formation rate with the change in the composition of the atmosphere obtained for the exhaust gas purifying catalyst according to Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described.

FIG. 1 is a diagram schematically showing an exhaust gas purifying catalyst according to an embodiment of the present invention. This exhaust gas purifying catalyst 1 is a pellet catalyst formed by agglomerating particles, and FIG. 1 shows one particle. This exhaust gas purification catalyst 1 is used under high temperature conditions of, for example, 1000 ° C to 1200 ° C.

The exhaust gas-purifying catalyst 1 shown in FIG. 1 includes a support 11, composite oxides 12 a to 12 c partially covered on the surface, and a noble metal 13 a supported on the support 11.

[0014] The support 11 contains a rare earth element oxide as a main component. The carrier 11 can further contain, for example, zircoure (ZrO 2). Carrier 11 is composed of rare earth elements and zirco.

2

-It may contain a complex oxide with um as the main component.

[0015] The composite oxide 12a contains a composite oxide of a rare earth element and an alkaline earth element as a main component. The composite oxide 12b contains a composite oxide of zirconium and an alkaline earth element as a main component. The composite oxide 12c contains a composite oxide of a rare earth element, zirconium and an alkaline earth element as a main component.

[0016] The rare earth elements contained in the composite oxides 12a and 12c are the same as the rare earth elements contained in the carrier 11, and the composite oxides 12a to 12c do not contain the same alkaline earth element. The The composite oxides 12a to 12c contain the same noble metal as the noble metal 13a and form a solid solution.

[0017] Here, as an example, the carrier 11 contains ceria (CeO) as a main component, and a composite oxide.

2

12a is composed of a complex oxide represented by the chemical formula: BaCeO, and complex oxide 12b is represented by the chemical formula: The complex acidity represented by BaZrO is the chemical formula: Ba (Zr, Ce) 0

3 It shall consist of the complex oxide represented by 3. Further, the noble metal and the noble metal 13a included in the composite oxides 12a to 12c are assumed to be platinum (Pt). That is, cerium is used as the rare earth element, norlium is used as the alkaline earth element, and platinum is used as the noble metal. The solid solution of the composite oxide 12a and platinum can be expressed, for example, by the chemical formula: Ba (Ce, Pt) O, and the solid solution of the composite oxide 12b and platinum can be expressed by the chemical formula: Ba (Zr, P

Three

t) 0, and the solid solution of the complex oxide 12c and platinum can be expressed, for example, by the formula: Ba (Zr,

Three

Ce, Pt) 0.

Three

[0018] This exhaust gas-purifying catalyst 1 exhibits an irreversible state change when the composition of the atmosphere is changed under high temperature conditions. This will be described with reference to FIG.

FIG. 2 is a diagram schematically showing a state change of the exhaust gas-purifying catalyst of FIG. 1 under high temperature conditions. In FIG. 2, the state indicated as “Lean” indicates that the exhaust gas purification is performed when exposed to a high oxygen concentration atmosphere under a high temperature condition of 1000 ° C. to 1200 ° C., for example, when the fuel supply to the engine is stopped. This shows the state of the catalyst 1 for metal. On the other hand, the state indicated as “Rich” is an exhaust gas when exposed to a low oxygen concentration atmosphere under a high temperature condition of 1000 ° C. to 1200 ° C., for example, when a large amount of fuel is continuously supplied to the engine. The state where the catalyst 1 for purifier is exhibited is shown.

[0020] The state indicated as "Lean" in FIG. 2 corresponds to the state described with reference to FIG. However, at this time, at least a part of the noble metal 13a may be oxidized (the oxidation number is increased).

[0021] In this state, the noble metal 13a mainly contributes to the activity of the exhaust gas purification catalyst 1, and platinum in the composite oxides 12a to 12c hardly contributes to the activity. However, during the period when the exhaust gas-purifying catalyst 1 is in the state of “Lean”, the concentration of harmful components (for example, nitrogen oxides, carbon monoxide, hydrocarbons, etc.) in the exhaust gas, that is, in the atmosphere The concentration of harmful components is relatively low. Therefore, the exhaust gas purifying catalyst 1 exhibits sufficient performance.

[0022] When the oxygen concentration in the atmosphere is lowered under the previous high temperature condition, the exhaust gas purifying catalyst 1 changes from the state indicated as "Le &11" to the state indicated as "1 ^ 11". Specifically, complex acid Platinum is deposited from the compounds 12a to 12c, and the deposited platinum forms a noble metal 13b on the surfaces of the composite oxides 12a to 12c.

[0023] During the period when the exhaust gas purifying catalyst 1 is in the state of "Rich", the concentration of harmful components in the exhaust gas is relatively high. That is, in the period corresponding to the state indicated as “Rich”, the exhaust gas-purifying catalyst 1 is required to have higher activity than the period corresponding to the state indicated as “Lean”.

[0024] The noble metal 13b is much smaller than the noble metal 13a. For example, the dimension of the noble metal 13a is about several lOnm, while the dimension of the noble metal 13b is several nm or less. Therefore, the exhaust gas purifying catalyst 1 that exhibits the state of “Richj” has higher activity than the exhaust gas purifying catalyst 1 that exhibits the state of “Lean”. is doing. Therefore, the exhaust gas purifying catalyst 1 exhibits sufficient performance even when the concentration of harmful components in the exhaust gas is high.

[0025] Exhaust gas purifying catalyst 1 exhibiting a state indicated as "Rich" changes to a state indicated as "Lean" when the oxygen concentration in the atmosphere increases under the previous high-temperature conditions. To do. That is, the platinum forming the noble metal 13b and the composite oxide form a solid solution. Platinum and silica rarely form a solid solution.

[0026] Thus, the exhaust gas purifying catalyst 1 undergoes a reversible state change. The exhaust gas-purifying catalyst 1 forms extremely fine noble metal 13b on the surfaces of the composite oxides 12a to 12c each time it changes from the state described as “Lean” to the state expressed as “Rich”. To do. Therefore, this state is recovered by causing a change from a state indicated as “Rich” to a state indicated as “Lean” and vice versa. In auto-propelled vehicles, the oxygen concentration in the exhaust gas changes relatively frequently, so this exhaust gas purification catalyst 1 always exhibits high activity and sufficient performance when exposed to a low oxygen concentration atmosphere at high temperatures. Demonstrate.

[0027] In the exhaust gas purifying catalyst 1, the noble metal 13a contributes to the activity of the exhaust gas purifying catalyst 1 regardless of the composition and temperature of the atmosphere. Therefore, the exhaust gas-purifying catalyst 1 exhibits sufficient performance when exposed to an atmosphere of high oxygen concentration at high temperatures, and also exhibits sufficient performance during initial use and low-temperature conditions.

[0028] Further, in the exhaust gas purifying catalyst 1, as described above, oxygen in the atmosphere under high temperature conditions. As the concentration increases, the noble metal 13b and the composite oxides 12a to 12c form a solid solution. For this reason, this exhaust gas purification catalyst 1 has a small evaporation loss of platinum in a high oxygen concentration atmosphere.

[0029] Although the case where cerium is used as the rare earth element has been described above as an example, other elements may be used as the rare earth element. For example, lantern, praseodymium, neodymium, etc. may be used. A plurality of rare earth elements may be used.

[0030] Elements other than norium may be used as the alkaline earth element. For example, strontium, calcium and magnesium may be used. A plurality of alkaline earth elements may be used.

[0031] Elements other than platinum may be used as the noble metal. For example, platinum group elements such as palladium and rhodium may be used. A plurality of noble metals may be used.

[0032] In the exhaust gas purification catalyst 1, the atomic ratio of the alkaline earth element to the sum of the rare earth element and zirconium is 10 atomic% or more, and typically 20 atomic% or more. The atomic ratio is, for example, 100 atomic% or less, and typically 80 atomic% or less.

When this atomic ratio is small, the volume ratio of the composite oxide 12 to the support 11 is small. Therefore, the performance recovery of the exhaust gas purifying catalyst 1 due to the change in the composition of the atmosphere may be insufficient. Further, when this atomic ratio is excessively increased, the ratio of the noble metal 13a to the total noble metal supported by the exhaust gas purification catalyst 1 is reduced. Therefore, sufficient catalytic activity may not be obtained under high temperature and high oxygen concentration conditions. If the atomic ratio is excessively increased, the heat resistance of the support 11 is lowered during high temperature use, and as a result, noble metal sintering may occur easily.

[0034] The precious metal content of the exhaust gas purification catalyst 1 is, for example, in the range of 0.01% by mass to 10% by mass, and typically in the range of 0.1% by mass to 5% by mass. If the noble metal content is low, sufficient catalytic activity may not be obtained. When the noble metal content is high, sintering of the noble metal may occur easily.

[0035] The ratio of the precious metal forming the solid solution to the total precious metal carried by the exhaust gas purification catalyst 1 (hereinafter referred to as the solid solution formation rate) is, for example, within a range of 10% to 80%. And When the solid solution formation rate is small, the decrease in activity due to sintering of noble metals is suppressed. The effect may be insufficient. When the solid solution formation rate is large, the initial activity may be insufficient.

[0036] The exhaust gas purifying catalyst 1 can be produced, for example, by the following method.

First, a powdery support 11 containing a complex oxide of a rare earth element and zirconium oxide as a main component is prepared, and a slurry thereof is prepared. At this time, for example, water is used as the dispersion medium. Next, a noble metal salt solution is added to the slurry and this is filtered. Subsequently, the filter cake is sequentially dried and fired. In this way, the support 11 is loaded with the noble metal.

[0037] Next, the carrier 11 supporting the noble metal is added to the alkaline earth salt solution. Sarako, this slurry is heated to sufficiently remove the liquid. In this way, the alkaline earth element is supported on the support 11.

[0038] There is no particular limitation on the method of supporting the alkaline earth element on the support 11. For example, a method of impregnating a carrier 11 supporting a noble metal with an alkaline earth salt solution, a method using coprecipitation, a method using an alkoxide of an alkaline earth metal, or the like may be used.

[0039] Thereafter, the carrier 11 carrying the noble metal and the alkaline earth element is fired in an oxidizing atmosphere. Thus, the composite oxides 12a to 12c are generated, and a solid solution of the composite oxides 12a to 12c and the noble metal is generated, and the particles shown in FIG. 1 are obtained.

[0040] Further, the fired powder is compression-molded, and the molded product is pulverized as necessary. As described above, a pellet-shaped exhaust gas purification catalyst 1 is obtained.

[0041] In this method, the firing temperature is, for example, in the range of about 700 ° C to about 1100 ° C. When the firing temperature is low, it is difficult to form noble metals in these composite oxides 12a to 12c, which are difficult to produce the composite oxides 12a to 12c. When the calcination temperature is high, the specific surface area of the carrier 11 decreases, and accordingly, it becomes difficult to satisfactorily disperse the noble metal 13a on the carrier 11. Therefore, high activity may not be obtained.

As described above, the case where the exhaust gas purifying catalyst 1 is a pellet catalyst has been described as an example, but the exhaust gas purifying catalyst 1 can take various forms. For example, the exhaust gas purifying catalyst 1 may be a monolith catalyst.

[0043] Examples of the present invention will be described below.

[0044] (Example 1) Cerium nitrate [Ce (NO)] and zirconium oxynitrate [ZrO (NO)] and cerium

3 3 3 2

They were weighed so that the atomic ratio with zirconium was 50:50, and these were added to 500 mL of deionized water. After thorough stirring, a 10% by mass aqueous solution of ammonium hydroxide was added dropwise to the aqueous solution at room temperature to cause coprecipitation. The aqueous solution containing the precipitate was stirred for 60 minutes and then filtered.

[0045] Next, the filter cake was thoroughly washed with deionized water and dried at 110 ° C. This dried product was subjected to pre-baking at 500 ° C for 3 hours in an air atmosphere. The obtained pre-baked product was pulverized in a mortar and further subjected to main baking at 800 ° C. for 5 hours in an air atmosphere.

[0046] The powder thus obtained was measured for a diffraction spectrum by an X-ray diffractometer. As a result, it can be confirmed that this powder has an acidity represented by the chemical formula: (Ce, Zr) 0.

2

It was. The specific surface area of this powder was 90 m ² / g.

[0047] Next, 50 g of the above oxide powder was weighed and added to 500 mL of deionized water.

The oxide powder was sufficiently dispersed in deionized water by performing ultrasonic stirring for 10 minutes, and then a dinitrodiammine platinum nitric acid solution was added to the slurry. The concentration and addition amount of the dinitrodiammine gold nitric acid solution were adjusted so that the amount of platinum supported was 1% by mass over the exhaust gas purification catalyst as the final product.

[0048] Thereafter, the slurry was subjected to suction filtration. The filtrate was subjected to inductively coupled radio frequency plasma (ICP) spectroscopy analysis and found that almost all of the platinum in the slurry was present in the filter cake.

[0049] Next, the filter cake was dried at 110 ° C for 12 hours. Subsequently, this was fired at 500 ° C. in the atmosphere. As a result, platinum was supported on the above oxide.

[0050] Thereafter, barium acetate was dissolved in lOOmL of deionized water. Next, 50 g of an oxide containing platinum was weighed and added to an aqueous barium acetate solution. The concentration of the barium acetate aqueous solution was adjusted so that the atomic ratio of barium to the sum of cerium and zirconium was 10.0 atomic% in the exhaust gas purification catalyst as the final product.

[0051] Next, this slurry was heated to remove excess water. Subsequently, this was calcined in the atmosphere at 1 000 ° C. for 3 hours. As a result, a complex oxide containing barium was produced, and a solid solution of the complex oxide and platinum was formed. [0052] The powder obtained as described above was measured for a diffraction spectrum by an X-ray diffractometer. The results are shown in Fig. 3.

FIG. 3 is a graph showing an X-ray diffraction spectrum of the powder produced in Example 1. In the figure, the horizontal axis indicates the diffraction angle, and the vertical axis indicates the diffraction intensity. As shown in Fig. 3, this powder has a chemical formula: (Ce, Zr) 0 in addition to a composite oxide represented by chemical formula: BaCeO.

twenty three

Oxide, chemical formula: Composite oxide represented by BaZrO, Chemical formula: Ba (Zr, Ce) 0

3 3 containing complex oxide!

[0054] A part of the powder obtained as described above was extracted and immersed in a 10% hydrogen fluoride aqueous solution maintained at room temperature for 12 hours. This condition is a condition in which only the complex oxide containing norium in the previous powder dissolves. Subsequently, this liquid was filtered, and the filtrate was subjected to ICP spectroscopic analysis. As a result, it was found from the platinum content of the filtrate that 45% of platinum formed a solid solution, that is, the solid solution formation rate was 45%.

[0055] Next, the above powder was compression molded. Further, this molded product was pulverized to obtain a pellet-shaped exhaust gas purifying catalyst having a particle size of about 0.5 mm to about 1. Omm.

[0056] (Example 2)

Exhaust gas was treated in the same way as described in Example 1 except that the concentration and amount of barium acetate aqueous solution were adjusted so that the atomic ratio of norium to the sum of cerium and zirconium in the final product was 30 atomic%. A catalyst for purifier was produced.

In this example, the amount of platinum supported and the solid solution formation rate were examined by the same method as described in Example 1. As a result, in this example, the amount of platinum supported was 1% by mass and the solid solution formation rate was 65%.

[0058] (Example 3)

Exhaust gas was exhausted by the same method as described in Example 1 except that the concentration and amount of barium acetate aqueous solution were adjusted so that the atomic ratio of norium to the sum of cerium and zirconium in the final product was 50 atomic%. A catalyst for purifier was produced.

In this example as well, the amount of platinum supported and the solid solution formation rate were examined by the same method as described in Example 1. As a result, in this example, the amount of platinum supported was 1% by mass and the solid solution formation rate was 70%. [0060] (Example 4)

Except that the concentration and amount of barium acetate aqueous solution were adjusted so that the atomic ratio of norium to the sum of cerium and zirconium in the final product was 80 atomic%, the same method as described in Example 1 was used. An exhaust gas purification catalyst was produced.

In this example as well, the amount of platinum supported and the solid solution formation rate were examined by the same method as described in Example 1. As a result, in this example, the amount of platinum supported was 1% by mass and the solid solution formation rate was 85%.

[0062] (Example 5)

Exhaust gas was exhausted by the same method as described in Example 1 except that the concentration and amount of barium acetate aqueous solution were adjusted so that the atomic ratio of norium to the sum of cerium and zirconium in the final product was 100 atomic%. A catalyst for purifier was produced.

[0063] In this example as well, the amount of platinum supported and the solid solution formation rate were examined by the same method as described in Example 1. As a result, in this example, the amount of platinum supported was 1% by mass and the solid solution formation rate was 85%.

[0064] (Comparative example)

Exhaust gas purification catalyst was produced in the same manner as described in Example 1 except that the addition of the platinum-supported oxide to the barium acetate aqueous solution and subsequent firing was not performed. .

[0065] In this example as well, the amount of platinum supported was examined by the same method as described in Example 1. As a result, in this example, the platinum loading was 1% by mass.

[0066] (Example 6)

In this example, first, an oxide powder represented by the chemical formula: (Ce, Zr, Y) 0 is converted by the following method.

2

Generated.

[0067] That is, first, cerium nitrate [Ce (NO)] and zirconium oxynitrate [ZrO (NO)]

3 3 3 2

] And yttrium nitrate [Y (NO)], the atomic ratio of cerium, zirconium and yttrium

3 3

These were weighed so as to be 45: 50: 5, and added to 500 mL of deionized water. After thorough stirring, a 10% by mass aqueous solution of ammonium hydroxide was added dropwise to the aqueous solution at room temperature to cause coprecipitation. The aqueous solution containing the precipitate was stirred for 60 minutes and then filtered. [0068] Next, the filter cake was thoroughly washed with deionized water and dried at 110 ° C. This dried product was subjected to pre-baking at 500 ° C for 3 hours in an air atmosphere. The obtained pre-baked product was pulverized in a mortar and further subjected to main baking at 800 ° C. for 5 hours in an air atmosphere.

[0069] The powder thus obtained was measured for a diffraction spectrum by an X-ray diffractometer. As a result, it was confirmed that this powder also has the acidity represented by the chemical formula: (Ce, Zr, Y) 0

2

did it. The specific surface area of this powder was 90 m ² Zg.

[0070] Next, this oxide powder was used instead of the oxide powder represented by the chemical formula: (Ce, Zr) 0.

2

Except that the concentration and amount of barium acetate aqueous solution were adjusted so that the atomic ratio of norium to the sum of cerium and zirconium in the final product was 30 atomic%. A catalyst for exhaust gas purification was produced by the method described above.

[0071] In this example as well, the amount of platinum supported and the solid solution formation rate were examined by the same method as described in Example 1. As a result, in this example, the amount of platinum supported was 1% by mass and the solid solution formation rate was 80%.

[0072] (Example 7)

In this example, first, an oxide powder represented by the chemical formula: (Ce, Zr, La, Nd) 0 is

2

Generated by the method.

[0073] That is, first, cerium nitrate [Ce (NO)] and zirconium oxynitrate [ZrO (NO)]

3 3 3 2

], Lanthanum nitrate [La (NO)] and neodymium nitrate [Nd (NO)], cerium and zircoyu

3 3 3 3

The atomic ratio of lanthanum, lanthanum, and neodymium was weighed to 50: 35: 10: 5, and these were added to 500 mL of deionized water. After sufficiently stirring, at room temperature, a 10% by mass aqueous solution of ammonium hydroxide was added dropwise to the aqueous solution to cause coprecipitation. The aqueous solution containing the precipitate was stirred for 60 minutes and then filtered.

[0074] Next, the filter cake was thoroughly washed with deionized water and dried at 110 ° C. This dried product was subjected to pre-baking at 500 ° C for 3 hours in an air atmosphere. The obtained pre-baked product was pulverized in a mortar and further subjected to main baking at 800 ° C. for 5 hours in an air atmosphere.

[0075] The powder thus obtained was measured for a diffraction spectrum by an X-ray diffractometer. As a result, this powder has the following chemical formula: (Ce, Zr, La, Nd) 0

2

It could be confirmed. The specific surface area of this powder was 90 m ² Zg. [0076] Next, an exhaust gas purification catalyst was produced by the same method as described in Example 6 except that this acid oxide powder was used.

In this example as well, the amount of platinum supported and the solid solution formation rate were examined by the same method as described in Example 1. As a result, in this example, the amount of platinum supported was 1% by mass and the solid solution formation rate was 70%.

[0078] Next, the durability of these exhaust gas-purifying catalysts was examined by the following method.

First, each exhaust gas purification catalyst was placed in a flow-type durability test apparatus, and a gas containing nitrogen as a main component was passed through the catalyst bed at a flow rate of lOOmLZ for 30 hours. During this time, the catalyst bed temperature was maintained at 1050 ° C. In addition, as the gas to be circulated through the catalyst bed, a lean gas obtained by adding 5% oxygen to nitrogen and a rich gas obtained by adding 10% carbon monoxide and nitrogen to nitrogen are used, and these gases are used every 5 minutes. Switched to.

[0079] Thereafter, these exhaust gas-purifying catalysts were placed in an atmospheric pressure fixed bed flow reactor. Next, while circulating the model gas through the catalyst bed, the catalyst bed temperature was raised from 100 ° C to 500 ° C at a rate of 12 ° CZ, and the exhaust gas purification rate during that time was continuously measured. . As the model gas, a gas having stoichiometric equivalents of oxidizing components (oxygen and nitrogen oxides) and reducing components (carbon monoxide, hydrocarbons, hydrogen) was used. The results are shown in the table below.

[table 1]

In the above table, the columns labeled “Ce”, “Zr”, “Y”, “La”, “Nd” indicate cerium, zirconium, yttrium in the metal elements other than platinum contained in the exhaust gas purification catalyst. The atomic ratios of lanthanum and lanthanum are listed. In the column labeled “1 ^”, V is the mass ratio of platinum in the exhaust gas purification catalyst. The column labeled “AEZ (RE + Zr)” shows the altitude for the sum of rare earth elements and zirconium in exhaust gas purification catalysts. The atomic ratio of the potash earth element (in this case, norium) is indicated. The column labeled “50% purification temperature” shows the lowest temperature of the catalyst bed that could purify 50% or more of each component in the model gas. “HC” and “NO” indicate hydrocarbon and nitrogen oxide, respectively.

[0081] As shown in this table, the exhaust gas purifying catalyst according to Examples 1 to 7 can purify the model gas at a lower temperature than the exhaust gas purifying catalyst according to the comparative example. did it. From these results, it can be seen that the exhaust gas purifying catalysts according to Examples 1 to 7 are superior in durability to the exhaust gas purifying catalyst according to the comparative example.

[0082] Next, the exhaust gas purifying catalyst according to Example 2 was again placed in a flow-type durability test apparatus, and the above-described lean gas was allowed to flow therethrough. Subsequently, the gas flowing through the catalyst bed was switched to the lean gas force rich gas described above. During this time, the temperature of the catalyst bed was maintained at 1050 ° C. Thereafter, the temperature of the catalyst bed was lowered while allowing rich gas to flow through the catalyst bed. After the temperature of the catalyst bed became sufficiently low, the exhaust gas-purifying catalyst was observed with a transmission electron microscope (TEM). Figure 4 shows this TEM image.

FIG. 4 is a TEM photograph of the exhaust gas purifying catalyst according to Example 2. As shown in Fig. 4, a large amount of platinum (Pt) is deposited on the composite oxide containing normium, and these platinum have extremely small dimensions. As described above, in the exhaust gas purifying catalyst according to Example 2, a large amount of very fine platinum was present on the composite oxide immediately after switching the flow gas from lean gas to rich gas under high temperature conditions.

[0084] Next, the exhaust gas purifying catalyst according to Example 2 was placed in a flow-type durability test apparatus, the temperature of the catalyst bed was set to 1050 ° C, and the above-described lean gas was allowed to flow therethrough. . Subsequently, the temperature of the catalyst bed was lowered while lean gas was circulated through the catalyst bed. After the temperature of the catalyst bed has become sufficiently low, a part of the exhaust gas purification catalyst is extracted, and the diffraction spectrum is measured with an X-ray diffractometer, as described in Example 1. The solid solution formation rate was investigated by the method of

[0085] Next, the catalyst bed containing the remaining exhaust gas purification catalyst was heated to 1050 ° C, and the rich gas was circulated through the catalyst bed. Subsequently, the temperature of the catalyst bed was lowered with rich gas flowing through the catalyst bed. After the temperature of the catalyst bed has become sufficiently low, a part of the exhaust gas purification catalyst is extracted and removed. In addition to measuring the diffraction spectrum with an X-ray diffractometer, the solid solution formation rate was examined by the same method as described in Example 1.

[0086] Thereafter, the catalyst bed containing the remaining exhaust gas purification catalyst was heated to 1050 ° C, and the above-described lean gas was circulated through the catalyst bed. Subsequently, the temperature of the catalyst bed was lowered while lean gas was circulated through the catalyst bed. After the temperature of the catalyst bed has become sufficiently low, a part of the exhaust gas-purifying catalyst is extracted, and the diffraction spectrum is measured with an X-ray diffractometer and the same method as described in Example 1 is used. The solid solution formation rate was examined.

FIG. 5 is a graph showing changes in the X-ray diffraction spectrum accompanying the change in the composition of the atmosphere obtained with the exhaust gas purifying catalyst according to Example 2. In the figure, the horizontal axis indicates the diffraction angle, and the vertical axis indicates the detection intensity. In the figure, curve A shows the diffraction spectrum immediately after the first flow of lean gas, curve B shows the diffraction spectrum immediately after the rich gas flow, and curve C shows the diffraction spectrum immediately after the lean gas flow again. The diffraction spectrum at is shown.

[0088] FIG. 5 shows, as an example, a peak derived from a complex acid compound represented by the chemical formula: BaZrO.

Three

I'm drawing. As shown in this figure, it is derived from the complex oxide represented by the chemical formula: BaZrO.

Three

The peak position shifted to the low angle side by switching the flow gas from lean gas to rich gas, and shifted to the high angle side by switching the flow gas from rich gas to lean gas. The peak position shifted to the low angle side by switching the flow gas from lean gas to rich gas again. From this, it can be concluded that the above complex oxide shows a reversible state change in accordance with the change in the atmosphere.

[0089] FIG. 6 is a graph showing the change in the solid solution formation rate with the change in the composition of the atmosphere obtained by the exhaust gas purifying catalyst according to Example 2. In the figure, the data indicated by “oxidation” indicates the solid solution formation rate measured immediately after the first flow of lean gas, and the data indicated by “reduction” indicates the solid solution formation measured immediately after the rich gas flow. The data shown in “Reoxidation” indicates the solid solution formation rate measured immediately after recirculating lean gas.

As is clear from FIG. 6, the exhaust gas purifying catalyst according to Example 2 generates a solid solution of the composite oxide and platinum by switching the flow gas to rich gas power lean gas at a high temperature. , By switching the flow gas to lean gas rich gas at high temperature, platinum was precipitated from the complex oxide. The same test was performed on the exhaust gas purifying catalysts according to Examples 1 and 3 to 7, and similar results were obtained. That is, the exhaust gas purifying catalysts according to Examples 1 and 3 to 7 also generate a solid solution of a composite oxide and platinum by switching the flow gas from rich gas to lean gas at high temperature, and flow at high temperature. By switching the gas from lean gas to rich gas, the precipitation of platinum with high complex acid strength occurred.

Further benefits and variations are readily apparent to those skilled in the art. Therefore, the present invention should not be limited to the specific descriptions and representative embodiments described herein in its broader aspects. Accordingly, various modifications may be made without departing from the true spirit or scope of the present invention as defined by the appended claims and their equivalents.

Claims

The scope of the claims

[I] It contains rare earth elements, alkaline earth elements, zirconium and noble metals, and the atomic ratio of alkaline earth elements to the sum of rare earth elements and zirconium is 10 atomic% or more. Part of the zirconium and part of the zirconium form a composite oxide with at least a part of the alkaline earth element, and the composite oxide and a part of the noble metal form a solid solution.

[2] The exhaust gas-purifying catalyst according to claim 1, comprising cerium as a rare earth element.

[3] The exhaust gas purifying catalyst according to claim 2, further comprising a rare earth element other than cerium.

4. The exhaust gas purifying catalyst according to claim 2, further comprising yttrium as a rare earth element.

[5] The exhaust gas purifying catalyst according to claim 3, comprising norlium as an alkaline earth element.

6. The exhaust gas purifying catalyst according to claim 5, wherein the atomic ratio is 100 atomic% or less.

7. The exhaust gas purifying catalyst according to claim 6, wherein the content of the noble metal is in the range of 0.01 mass% to 10 mass%.

8. The exhaust gas purifying catalyst according to claim 7, wherein 10% to 80% of the noble metal forms the solid solution.

[9] The exhaust gas purifying catalyst according to claim 2, comprising norlium as an alkaline earth element.

10. The exhaust gas purifying catalyst according to claim 9, wherein the atomic ratio is 100 atomic% or less.

[II] The exhaust gas hatching catalyst according to claim 10, wherein the content of the noble metal is in the range of 0.01% by mass to 10% by mass.

12. The exhaust gas hatching catalyst according to claim 11, wherein 10% to 80% of the noble metal forms the solid solution.

[13] The exhaust gas purifying catalyst according to claim 1, comprising norlium as an alkaline earth element.

14. The exhaust gas purifying catalyst according to claim 1, wherein the atomic ratio is 100 atomic% or less.

15. The exhaust gas purifying catalyst according to claim 1, wherein the content of the noble metal is in the range of 0.01% by mass to 10% by mass.

16. The exhaust gas purifying catalyst according to claim 1, wherein 10% to 80% of the noble metal forms the solid solution.